1. Field of the Invention
The present invention relates to a motor control device that drives and controls a motor.
2. Description of Related Art
In order to perform a vector control of a motor by supplying three-phase AC power to the motor, it is necessary to detect current values of two phases (e.g., U-phase current and V-phase current) among three phases including U-phase, V-phase and W-phase. Although two current sensors (current transformers or the like) are usually used for detecting current values of two phases, the use of two current sensors causes an increase of cost of the entire system equipped with the motor.
For this reason, there is provided a conventional method in which bus current (DC current) between an inverter and a DC power supply is sensed by a single current sensor, and current values of two phases are detected from the sensed bus current. This method is also called a single shunt current detecting method.
In the single shunt current detecting method, a carrier signal like a triangular wave in PWM (Pulse Width Modulation) is compared with a specified voltage value for each phase voltage, so that switching elements in the inverter are switched at timings when the carrier signal cross the specified voltage values. Then, timing when current of a minimum phase and a maximum phase of the voltage flows as bus current is calculated based on the specified voltage values, and the three phase current values are detected from bus current sampled at the timing.
However, it is necessary to calculate the sampling timing of the current sensor accurately in the single shunt current detecting method. In addition, if two phase voltage values among three phase voltage values become the same value, it is impossible to detect current values of two phases in a single carrier period. If the current values of two phases cannot be detected, the three phase current values cannot be detected. Therefore, vector control of a motor cannot be performed.
For this reason, there is proposed a method of detecting phase current of one of the three phases by using a single current sensor so as to estimate other two phase current values or d-axis current and q-axis current.
For example, in a first conventional method, d-axis current and q-axis current are calculated from a current value of the detected phase current of one phase by using a state equation, so as to perform the vector control.
Furthermore, in the second conventional method, the dq conversion is performed on detected phase current of one phase and estimated phase current of other two phases so that torque current and exciting current are estimated. Then, the torque current and the exciting current are equalized by using a first order lag filter, and the inverse dq conversion is performed on them so as to estimate the above-mentioned other two phase current, which are fed back to the input side.
Furthermore, in the third conventional method, the bus current (DC current) between the DC power supply and the inverter is sensed, and the sensed bus current is converted directly into q-axis current by using an equation concerning electric power. More specifically, noting the fact that “the product of the sensed bus current and DC voltage from the DC power supply” is equal to “the sum of the product of the d-axis current and the d-axis voltage and the product of the q-axis current and the q-axis voltage” (or they have a proportional relationship), the q-axis current is calculated from the sensed bus current.
Furthermore, in the fourth conventional method, one phase current is detected and a rotor position of a motor is detected. Then, an amplitude of a current and a U-phase current phase angle are calculated by using a rotor position when the detected phase current becomes zero, and other two phase currents are calculated by using a result of the above-mentioned calculation.
Furthermore, in the fifth conventional method, if a difference between two phase voltages is small in the single shunt current detecting method, a carrier frequency of the PWM is decreased. If detection of the phase current is still difficult, three phase current values are estimated from past current information. More specifically, the d-axis current and the q-axis current obtained by converting the past three phase current values are converted inversely into three phases, so that the three phase current values are estimated.
In this way, the technique of performing vector control by using a single current sensor is beneficial to cost reduction or the like, but it is necessary to devise a method corresponding to reduction of numbers of current sensors.